This invention relates to a nest level judging device for use in a protocol conversion system.
A protocol conversion system of the type described, is for use in an information communication network between heterogeneous data processing systems. The information communication network between heterogeneous data processing systems is for mutually connecting between the heterogeneous data processing systems in a local area or between local areas to carry out information interchange between the heterogeneous data processing systems. Such an information communication network is expected to spread wide in society of information. In general, the information communication network is built on the basis of a network architecture systematized according to a protocol.
Inasmuch as a such protocol has been originally developed by each of computer makers, it is impossible to carry out the information interchange between the heterogeneous data processing systems. In order to solve this problem, a standard protocol has been established by International Organization for Standardization (ISO). This standard protocol is called a layer protocol of Open Systems Interconnection (OSI) in the art. Such a layer protocol is disclosed in a book written by William Stallings under the title of "DATA AND COMPUTER COMMUNICATIONS", pages 394 to 408, Chapter 12.
The layer protocol consists of seven layers which comprise a physical layer, a data link layer, a network layer, a transport layer, a session layer, a presentation layer, and an application layer. Each layer of the layer protocol has a processing function which should be standardized between the computer makers. Inasmuch as the layer protocol of the OSI is well-defined, it is possible to carry out protocol conversion between the layer protocol and a specific protocol developed by each computer maker. Each of the heterogeneous data processing systems comprises a protocol conversion system which carries out the protocol conversion. By the protocol conversion, it is possible to carry out the information interchange between the heterogeneous data processing systems.
Attention will be directed to the application layer. As is known in the art, the application layer is divisible into first through N-th sublayers where N represents a predetermined natural number which is equal to, for example, sixty-four. The first through the N-th sublayers are referred to first through N-th nest levels, respectively, in the art. The first through the N-th nest levels are a lowest through a highest nest level, respectively. According to the application layer, data transmission is carried out between the heterogeneous data processing systems on the basis of Message Handling Systems (MHS) recommended by the International Telegraph and Telephone Consultive Committee (CCITT) Recommendation X.409. More specifically, data transmitted between the heterogeneous data processing systems are a plurality of tokens. Each of the tokens is given a nest level selected from the first through the N-th nest levels as a selected level. Each of the tokens comprises a header and a data set as described in the Chapter 12 of the above-mentioned book. The data set has a data length and is for nesting one of the tokens that has a higher nest level than the selected level and will be referred to as a particular token. The header comprises a data length code representative of the data length and a data identifier code. The data identifier code includes a nest bit indicative of whether or not the particular token is nested in the data set.
In the protocol conversion system, the selected level must be judged. A conventional method for judging the selected level has been performed by software in computers. According to the conventional method, the tokens are memorized in a main memory as memorized tokens. Codes of the respective memorized tokens are successively read from the main memory to a central processing unit (CPU) as read codes. The software is used in judging the selected level from the read codes. The conventional method has been therefore disadvantageous in that a large processing time is consumed to judge the selected level. As a result, it is impossible to rapidly and smoothly carry out the information interchange.